This paper studies Galileo Galilei’s Trattato della sfera ovvero cosmografia, which was written before 1600. It is a geocentric astronomical treatise that follows the main structure of Johannes de Sacrobosco’s Tractatus de sphæra. This paper analyzes some peculiarities of Galileo’s treatise, comparing it to several other vernacular astronomical works of the sixteenth century and discussing its likely sources. Contrary to previous claims, we argue that Christoph Clavius’ commentary on Sacrobosco’s Sphæra cannot be regarded as its only or main influence. A likely inspiration for Galileo’s treatise is Alessandro Piccolomini’s Sfera del mondo, a work that anticipated several specific features of the Trattato della sfera. This influence is corroborated by Galileo’s copious marginal notes found in a copy of Piccolomini’s book.

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Outline

1 Introduction

2 Comparison between the Trattato della sfera and Sacrobosco’s Tractatus

3 Language and aim of the Trattato della sfera

4 Suggested sources of the Trattato della sfera

5 Sixteenth-century Italian astronomical works

6 Galileo’s annotations in Piccolomini’s Sfera

7 The influence of Piccolomini’s Sfera on Galileo

8 Other sources of the Trattato della sfera

9 Final remarks

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1 Introduction

One of the earliest works attributed to Galileo is called Trattato della sfera ovvero cosmografia [Treatise on the Sphere, or Cosmography]. Its date of composition is unknown (probably before 1600) and it was published posthumously by the priest Urbano d’Aviso [Galilei 1656]. It is a short and elementary geocentric astronomical treatise. Its content and structure generally follow Johannes de Sacrobosco’s medieval Tractatus de sphæra. A first look at the contents and style of the Trattato della sfera ovvero cosmografia (hereinafter called Trattato in brief) provides no internal evidence that it was written by Galileo. It includes no reference to Copernicus or his ideas; it accepts and defends the main geocentric astronomical ideas: that the Earth does not move in any way, that it is at the center of the universe, and that the Sun, the Moon, the planets and the stars move around it. Also, it contains no recent information that became available during the sixteenth centur...